Large-scale Seismic Experiments Conducted by Japan Coast Guard in the Northwestern Pacific Plate and the Philippine Sea Plate: ―海上保安庁による調査結果―

Japan Coast Guard conducted seismic reflection experiments with multi-channel streamer cable and refraction experiments with ocean bottom seismographs from 2004 to 2008, as part of continental shelf surveys, to clarify the crustal structures of various submarine topographic features around Japan. These provide useful information on their formation process and tectonics. The total number of survey lines approaches 100 and their total length is over 50,000 km. The total number of ocean bottom seismographs deployed for the experiments is over 5,200. Seven sea areas are established in the current research: 1) Minami-Tori Shima area, 2) Ogasawara Plateau area, 3) Japan Trench area, 4) Shikoku Basin and Parece Vela Basin area, 5) Kyushu. Palau Ridge area, 6) Daito Ridges area and 7) West Philippine Basin area. Characteristic velocity structures are shown in each area, as described below: 1) Characteristic topographic features around Minami-Tori Shima are seamounts formed by intraplate igneous activities on an oceanic crust. The largest seamount in this area, Takuyo-Daigo Seamount, has several intrusion cores and thickened crust. The Northwest Pacific Basin displays a typical oceanic crust with large velocity anisotropy in the uppermost mantle. 2) Ogasawara Plateau is located at the edge of the Pacific Plate, where it collides with the adjacent Philippine Sea Plate. The crustal thickness of the plateau is estimated to be over 20 km, which might make subduction below the trench difficult. 3) Erimo Seamount and Daiichi-Kashima Seamount are subducting below the North American Plate. Sections of plate boundary with the seamounts are obtained. 4) The Shikoku Basin and Parece Vela Basin are back-arc basins, formed by rifting of the proto Izu. Ogasawara Arc. The crustal thickness of these basins is similar to 5 km, which is thinner than that of the typical oceanic crust produced at a mid-ocean ridge (similar to 7-8 km). 5) The Kyushu-Palau Ridge is a remnant arc of the proto Izu-gasawara intra-oceanic island arc. Although crustal models of the Kyushu. Palau Ridge vary along the ridge, the crusts are signifi cantly thicker than those of oceanic basins and show similar characteristics to the Izu-Ogasawara arc crust. 6) The Daito Ridges are composed of the Amami Plateau, Daito Ridge and Oki-Daito Ridge. These bathymetric highs have a thicker crust at 15-25 km and also have same paleo-island arc features. 7) The West Philippine Basin is believed to spread from the CBF Rift. The crustal thickness of the basin is approximately 4-6 km, which is similar to that of the Shikoku Basin and Parece Vela Basin. However, the higher velocities of the lower crust and uppermost mantle are meaningfully different from the neighboring backarc basins.